Would it be churlish to point out Frankenstein doesn't do much revenge-taking here, any more than he was evil in The Evil of Frankestein, or was particularly cursed in... Well, you get the picture. What he does manages to do, is cheat the guillotine which awaited him at the end of Curse, with a priest taking the blade instead. The Baron (Cushing) quietly sets up shop in another town, attending to the rich, a success that brings him under the displeased scrutiny of the local medical council. He also does "charity work" at a hospital for the poor, which provides a handy source of spare parts for his off-hours research. Another local physician, Dr. Kleve (Matthews) recognises him, blackmailing his way into an assistant's position, and the pair work towards transplanting the brain of the Baron's deformed assistant Karl into a spiffy new body (Gwynn). Never mind that, when they tried it on a monkey, it turned cannibalistic. What could possibly go wrong?
Naturally, things do; what fun would a movie be, where the scientist's creation didn't get to go on the rampage? Interestingly, the "creation" here is, at least initially, largely indistinguishable from the rest of us. While it's a novel angle - monster is, as monster does, perhaps - the results aren't remotely terrifying, because the way Gwynn plays it, he comes off more as a belligerent drunk than a nightmarish creation of science gone horribly wrong. Against that, we have Cushing: he proved through the series why he is Baron Frankenstein sans pareil, and does the job again here. The Baron being a man oblivious to everything except his own goals; even when helping others, it's for selfish reasons of his own knowledge. After being called before the medical council and their suspicions as to his identity, as Kleve begs him to flee, Frankenstein completely refuses to cave to their pressure, in a way that's almost admirable. As so often with Hammer, Cushing is responsible for a huge part of the film's lasting quality and appeal, and his portrayal here fleshes out the Baron into a character with depth.